1. Field of the Invention
The invention relates to a deflection unit for a cathode ray tube having a neck portion and a display screen, the deflection unit being arranged between the neck portion and the display screen and around the flared portion of the tube connecting the neck portion and the display screen, the deflection unit comprising a field coil system and a line coil system for deflecting an electron beam produced in the neck portion in mutually orthogonal directions; the field coil system having a pair of diametrically opposite saddle type field deflection coils located on either side of a vertical axis and the line coil system having a pair of diametrically opposite saddle type line deflection coils located on either side of a horizontal axis extending at right angles to the vertical axis; each coil having a front end segment, a rear end segment and conductors extending between the front and the rear end segments.
2. Description of the Related Art
A deflection unit of the above described type is known from U.S. Pat. No. 4,229,720, issued Oct. 21, 1980, which corresponds to Netherlands patent specification No. 170,573 corresponding to U.S. Pat. No. 4,229,720, issued Oct. 21, 1988 and from the magazine "Funkschau" No. 23, 1980, pages 88-92 published in West Germany by Fanzis-Verlag GmbH published in West Germany.
In a deflection unit of this type the line deflection coils which generate a vertical magnetic field for the horizontal deflection must be arranged at right angles to the field deflection coils which generate a horizontal magnetic field for the vertical deflection. In the case of mutually orthogonal positions the magnetic coupling between the coil pairs is equal to zero so that no voltage is induced in the field deflection coils as a result of the magnetic field generated by the line deflection coils.
However, in practice it may occur that due to mechanical inaccuracies and/or manufacturing tolerances of the components during assembly the line deflection coils are not arranged exactly at right angles to the field deflection coils. In such a case a voltage will be induced in the field deflection coil as a result of the magnetic field of the line deflection coils. Detrimental consequences thereof are:
(a) the induced voltage reaches the field deflection circuit and the high voltage thus generated will disturb the operation of this field deflection circuit,
(b) the induced voltage produces a current through the field deflection coil via the field deflection circuit so that a rotation of the horizontal lines of the raster with respect to the horizontal axis becomes visible on the display screen. The convergence is also affected (twist errors).